I recently attempted to get a new site up and running with WordPress and the WP Super Cache plugin on Dreamhost. After configuring everything and being confident things were working just great, I started receiving errors at the bottom of the page that said “Page not cached by WP Super Cache. Could not get mutex lock.”
The FAQ page for WP Super Cache mentions how this is a file locking issue. A possible solution would be to modify the wp-content/wp-cache-config.php and uncomment the following line:
$use_flock = true
It also mentions setting $sem_id to a different value. So I changed the following line:
$sem_id = 5419;
$sem_id = 5420;
These steps didn’t work. So I was lost as to what to do next.
Fortunately the WordPress Forums proved useful with the solution. As dragonwize suggests, I set the “wp-content/cache/wp_cache_mutex.lock” to writable via FTP client and things started working once again.